Why are non jira users being added automatically as jira users and added as request participants to issues if service desk agents send an email to the jira email and CC non jira users?

[Mark Huey]

This question is in reference to Atlassian Documentation: Adding request participants

I am noticing that when our Service Desk agents send an email out to customers and CC other people that are not directly our customers and not users in our JIRA Service Desk, their email accounts are automatically added as JIRA users and they are added as request participants. 

I have made sure that Auto Email Signup is disabled, so I don't think this should be occurring. 

[Nic Brough -Adaptavist-]

Are they JIRA users, or set up as additional customers? 

JIRA needs at least a customer account to work with when adding participants (the field is a user field), so I'd expect it to create them.